Thanks for the question! So (C) tells us that the argument confuses the value of the object delivered with the value of delivering that object. Here, the “object delivered” is the letter. And the value of delivering that object is the value you get from the postal service for delivering the letter. The argument concludes that the postal service is undervalued because the things that it delivers (letters) are super valuable. So here, it’s confusing the value of the letters its delivering (super valuable) with the value of the delivery service itself (possibly not as super valuable, but we can’t conclude how valuable it is based on how valuable individual letters are).
Hope this helps! Feel free to ask any other questions that you might have.